The Internet comprises a vast number of computers and computer networks that are interconnected through communication links. The interconnected computers exchange information using various services, such as electronic mail, Gopher, and the World Wide Web (WWW). The WWW service allows a server computer system (i.e., Web server or Web site) to send graphical Web pages of information to a remote client computer system. The remote client computer system can then display the Web pages. Each resource (e.g., computer or Web page) of the WWW is uniquely identifiable by a Uniform Resource Locator (URL). To view a specific Web page, a client computer system specifies the URL for the Web page in a request (e.g., a HyperText Transfer Protocol (HTTP) request). These follow the familiar format http://www.example.com uniquely identifying the particular resource. The request is forwarded to the Web server that supports that Web page to the client computer system. When the client computer system receives that Web page, it typically displays the Web page using a browser. A browser is a special-purpose application program that effects the requesting of Web pages and the displaying of Web pages.
The domain name system (DNS) is the world's largest distributed computing system that enables access to any resource in the Internet by translating user-friendly domain names to IP Addresses. The process of translating domain names to IP Addresses is called Name Resolution. A DNS name resolution is the first step in the majority of Internet transactions. The DNS is in fact a client-server system that provides this name resolution service through a family of servers called Domain Name Servers. The hierarchical domain space is divided into administrative units called zones. A zone usually consists of a domain (say example.com) and possibly one or more sub domains (projects.example.com, services.example.com). The authoritative data needed for performing the name resolution service is contained in a file called the zone file and the DNS servers hosting this file are called the authoritative name servers for that zone. The DNS clients that make use of the services provided by authoritative name servers may be of two types. One type is called a stub resolver that formulates and sends a query every time it receives a request from an application that requires Internet service (e.g., a browser). The other type is called a caching (also called recursive/resolving) name server that caches the name resolution responses it has obtained from authoritative name servers and thus able to serve multiple stub resolvers.
The zone file hosted on an authoritative name server consists of various types of records called Resource Records (RRs). Associated with each DNS resource record is a type (RRtype). The code for these RRtypes is assigned by an international organization called Internet Assigned Names Authority (IANA). An RR of a given RRtype in a zone file provides a specific type of information. Some of the common RRtype codes are: NS, MX, CNAME, and A. An NS RR in a zone file gives the fully qualified domain name (FQDN) of the host that is considered the name server for that zone. For example, an NS RR in the zone file of the zone example.com may give the information that the host nsl.projects.example.com is a name server for the domain projects.example.com. Similarly an MX RR gives the host name for a mail server for the zone. An A RR gives the IP address for a host in a domain within the zone. CNAME provides “canonical name” records and maps names in the zone file. A zone file generally consists of multiple RRs of a given RRtype with some exceptions (e.g., there can be only SOA RR in a zone file). It can also have multiple RRs for the same domain name and same (or different) RRtype (e.g., multiple name servers or mail servers for a domain services.example.com).
The DNS infrastructure consists of many different types of DNS servers, DNS clients and transactions among/between these entities. The most important transaction in DNS is the one that provides the core service of DNS (i.e., name resolution service) and is called the DNS Query/Response. A DNS Query/Response transaction is made up of a query originating from a DNS client (generically called a DNS resolver) and response from a DNS name server. The response consists of one or more RRs. These RRs may be served from its own zone file (for an authoritative name server) or from a cache of RRs obtained from other name servers (for a caching/resolving/recursive name servers). In this way, the DNS serves as a global, distributed database. Name servers (serving zone files) each contain a small portion of the global domain space, and clients issue queries using a domain name and a desired RRtype.